A "synchro" is a generic term, which is used to cover electrical mechanical devices, equivalent to a variable transformer, which provide electrical output information proportional to an a.c. input voltage times some function of the angular displacement of its rotor winding to its stator windings. These devices can be made very accurate, to less than a minute of arc. However, to achieve this accuracy, it is required that the magnetic circuit consisting of the wound internal laminated core, normally called the rotor, the wound laminated structure surrounding it called the stator, and the air gap between them be very uniform, so that the magnetic permeability for a flux generated by windings on the rotor have very little change with rotor rotation. These devices, where high accuracy and stability of output information is required, are very sensitive to radial stresses which cause the rotor or stator to become out of round, which will change the air gap permeability and which can change the magnetic permeability of the stator and rotor cores.
Synchros are typically used to measure angular displacements between gimbals. In many of these applications where maximum accuracy is required, the synchro component with its bearings may support one end of the gimbal, where the rotor hub is fastened to one gimbal and the synchro housing to a fixed structure or another gimbal. Gimbals are typically made of aluminum in order to save weight, provide better thermal conductivity, and reduce costs. The synchro stator is mounted in a housing which matches the stator coefficient of expansion; and the synchro rotor is mounted to a hub which matches the rotor coefficient of expansion. There is a large difference between the coefficient of magnetic steel alloys used for the synchro and the aluminum gimbal. The synchro housing and rotor are tightly clamped to these mounting structures to prevent slippage due to shock and vibration. Consequently, as a result of changes in ambient temperatures and the difference in coefficient of expansion of the materials, the synchro housing is stressed. The temperature stress for large difference in ambient temperatures can be high enough to cause a slippage between the synchro housing and the aluminum gimbal, which will cause nonuniform and nonrepeatable radial distortions to the stator, thereby degrading its accuracy and the repeatability of its calibration.